Engine starting during cold operating conditions, referred to as a “cold start”, can present numerous challenges in maintaining repeatability/reliability and meeting emission requirements. Specifically, providing appropriate engine air-fuel ratio during engine starting conditions can be difficult due to numerous factors, especially given that exhaust gas oxygen sensors used for feedback air-fuel control are typically unavailable during the initial operation of a cold start. As such, the initial fueling may be referred to as open-loop air-fuel control.
One phenomenon that can degrade cold start air-fuel ratio control is when a portion of injected fuel may not be available for combustion due to fuel vaporization. This phenomenon may be referred to as “lost fuel” and can be significantly influenced by intake port surface temperature at start-up and fuel volatility (vapor pressure and distillation properties). Further, lost fuel can significantly impact open-loop fueling precision and accuracy, and cause the observed open-loop air-fuel ratio to deviate from the desired target value.
One approach to provide improved air-fuel ratio control is provided in U.S. Pat. No. 6,266,957. In this example, upon identifying activation of an air-fuel ratio sensor and when an absolute value of the deviation between a target air-fuel ratio and an actual air-fuel ratio is equal to or greater than a predetermined value, a correction value is calculated at that moment and used to update an existing value within the backup RAM.
However, the inventors herein have recognized a disadvantage with such an approach. In particular, the amount of correction at the exact moment of sensor activation may not accurately reflect the open-loop fueling error caused by lost fuel effects. Further, depending on the type of exhaust gas oxygen sensor provided, it may not be possible to identify how much error is present at the exact moment of sensor activation.
As such, one example approach to address the above issues uses a method for controlling fueling of an engine. The method comprises, during an engine cold start and before the engine is warmed to a predetermined level, transitioning from open-loop fueling to closed-loop fueling, where during closed-loop fueling feedback from an exhaust gas oxygen sensor is utilized and where said closed-loop fueling generates a cycling of delivered fuel in maintaining exhaust air-fuel ratio at a desired level; and providing a fueling adjustment to a subsequent engine start in response to fueling information, said fueling information obtained over at least a complete cycle of closed-loop fueling following said transition from open-loop fueling.
In this way, it is possible to utilize feedback information to obtain a more accurate determination of appropriate fueling during cold start open-loop conditions, thereby better accounting for variations in lost fuel. For example, as the engine ages, lost fuel can vary, thereby leading to increased emissions if not otherwise corrected.
In one particular aspect, by using cycle average information of first complete fueling cycle, it is possible to obtain ever more accurate fueling corrections. In another aspect, the fueling adjustment is provided only under select conditions to avoid inaccurate readings that may be caused by various conditions.